Communication techniques or energy transmission techniques, calculation and control processing techniques etc. using light are recently being given attention. For example, an optical fiber for connecting a plurality of communication devices, an optical inter-connection for connecting the print substrates, an optical circuit substrate for connecting a plurality of circuits through light, an optical integrated circuit (optical IC, OEIC (Opto Electronics Integrated Circuit) for performing information transmission using light in the circuit etc. have been developed. Since the large volume of data can be transmitted at high speed and the number of wirings can be reduced compared to the conventional device using electricity in addition to the functions of wiring and circuit using electricity of the prior art through the use of light, and the device can be miniaturized.
A waveguide for guiding the optical signal or the optical energy is used in communication and energy transmission that employs light. The waveguide is generally configured with a core, which is the portion having a refraction index higher than the upper and lower clad layers, formed between the lower clad layer and the upper clad layer, where the light that has entered the core is transmitted while being totally reflected at the interfaces of each clad layer and the core.
Conventionally, when manufacturing such waveguide, the core and the clad are formed by using quartz as the material and performing ion injection method, ion exchange process etc. to the quartz.
Furthermore, development in waveguide made from resin that excels in material cost is recently being advanced. However, when forming such waveguide, a core pattern using a photoresist is formed one sheet at a time, and processing is subsequently performed by reactive ion etching etc. in order to form the core configuration on the surface of the clad layer, and thus a semiconductor process using an expensive facility and device is required in the manufacturing steps, which thus increases the manufacturing cost.
A method is being reviewed of pressing a stamper having a convex shape of the core pattern of the waveguide against the polymer in the molten state or in the liquid state and curing the polymer in such state to transfer (copy) the groove of the core pattern thereby obtaining the lower clad layer.
When manufacturing the core groove of the waveguide through copying in manufacturing the waveguide using the polymer, that is, the waveguide made of resin, a release tapered shape must be formed at the cross sectional shape of the core so that the stamper can be easily released, which cross sectional shape of the core shape is generally a trapezoid instead of a rectangle. The shape of the waveguide manufactured using such method is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2003-240991 (Published on Aug. 27, 2003), which is a Japanese Laid-Open Patent Publication.
In recent years, a technique of replacing the conventional FPC (Flexible Printed Circuit) substrate, electrical wiring plate etc. by the film waveguide or the waveguide having flexibility is being developed.
However, in the waveguide in which the cross sectional shape of the core is a trapezoid or a rectangle, separation easily occurs due to the difference in coefficient of thermal expansion (thermal expansion coefficient) particularly between the core and the clad at the core-clad interface.
When the core and the clad separates, a microscopic chip forms at the core or microscopic clad resin attaches to the surface of the core thereby scattering the light propagated through the core at such microscopic portion and causing optical loss. In the single mode waveguide, the difference in the refraction index of the core and the clad at the separated portion changes, the amount of light that leaks out from the core to the clad changes, and optical loss occurs. Furthermore, if one portion separates, moisture enters from the relevant portion, separation advances due to change in temperature and the reliability further lowers.
The flexibility is desired for the film waveguide expected to replace the FPC substrate and the electrical wiring plate, but the core-clad interface tends to separate though repetitive bending.
The cause of this drawback is described using FIGS. 9(a) and 9(b). FIG. 9(a) shows cross sectional views of the waveguide which cross sectional shape of the core is a trapezoid and the waveguide which cross sectional shape of the core is a triangle. As shown in the figures, each waveguide includes a lower clad 101, an upper clad 102, and a core 103. The core 103 is formed by filling a material having a refraction index and the thermal expansion coefficient different from each clad in the groove formed in the lower clad 101. The material of both clad may be the same or may be different.
Since the thermal expansion coefficient of the core 103 and the thermal expansion coefficient of each clad are different, shearing force generates in between. The most general portion where the shearing force acts is the interface between the core 103 and the lower clad 101.
FIG. 9(b) shows cross sectional views showing the portion surrounded by a broken line in each waveguide shown in FIG. 9(a), that is, the interface between the core 103 and the lower clad 101. As shown in the figure, if the core shape is a trapezoid or a rectangle, the shearing force acts greatly on the interface due to the difference in the respective stretching force caused by the difference in the coefficient of linear expansion (thermal expansion coefficient), and separation occurs at the relevant location.